1. Field of the Invention
The present invention relates to a composite material of carbon-boron (hereinafter referred to as C-B) and, more particularly, to a method for producing a composite material in which boron component (hereinafter referred to as B) is evenly dispersed or distributed in the form of ultrafine particles in carbon component (hereinafter referred to as C) and to various uses of the composite material obtained by the method, in particular, uses as a neutron absorbent and as an oxidation resistant carbon material.
2. Description of Prior Art
C-B composite material has been heretofore popularly researched, developed and used as a neutron absorbent in atomic industry, as a carbon material having oxidation resistance and as a structural material for machines.
A conventionally employed method for producing the C-B composite material comprises the steps of mixing a B.sub.4 C (boron carbide) produced separately with either a carbon material or a material possible to be carbonized, baking them at high temperature, and treating them to be transformed into a solid solution, as is disclosed in Japanese Laid-Open Patent Publication (unexamined) No.1987-108767, Japanese Patent Application No.1987-297207, etc.
In the C-B composite material obtained according to the mentioned conventional process, B.sub.4 C is mixed with carbon after crushing and grinding a large and rough solid of B.sub.4 C. In this respect, because such grinding is mechanically performed, there is a limit in the pulverizing of B.sub.4 C. Accordingly, in a C-B composite material thus obtained, there are a B.sub.4 C portion, a solid solution portion of B.sub.4 C and carbon, and a portion of carbon alone. In effect, when observing microscopically, the obtained C-B composite material is not always completely even as a whole.
In the mentioned conventional method, because the two kinds of powder are mixed with each other, molded and baked, there arises a difficulty in the steps of cutting and machining the baked material, hence a problem exists from an economical point of view in that material productivity of expensive B.sub.4 C is considerably reduced by removal and waste of cut powder.
Moreover, a most serious problem in the conventional method exists in that a large amount of mineral impurities other than boron are included (usually 5000 ppm). Those impurities come mixed into the composite material due to contact with machines and equipment made of steel continuously during each step of grinding, mixing, molding and baking of compound material. This disadvantage is very difficult to overcome in the conventional methods. Coexistence of a small amount of mineral impurities may not be a serious problem depending upon the use of B-C composite material. However, when employing such a B-C composite material mixed with mineral impurities as a material for atomic industry, the mixed mineral impurities generate simultaneously reactions such as isotope transformation, splitting, etc. by radiation exposure or the like. Accordingly, when using it as a nuclear fusion material, a fatal disadvantage of decreasing plasma temperature due to high temperature elements emitted from the impurities will come out. Therefore, as a carbon material for use in atomic industry, a highly purified specific material, i.e. an ultrahighly pure material of which mineral impurity is not more than 5 ppm, more preferably not more than 2 ppm, substantially 0 ppm (measured by atomic absorption photometer) is usually employed. In a method for processing such an ultrahigh pure material, the mineral impurities are eliminated by halogenation treatment of high volatility, as is disclosed in the Japanese Patent Application No.61-224131 for example. In the composite material of B.sub.4 C-C, however, because of existence of boron in carbon material, impurities cannot be eliminated by using the mentioned treatment.
The mentioned disadvantage of the conventional method may result in a still further problem in that when using a crucible for melting alloy, stirring bars etc. necessary to obtain a specifically precious composition in metallurgical industry and the mentioned instruments are oxidated and worn out, then impurities may be mixed into the alloy eventually resulting in pollution.
Also when using the mentioned composite material as a member to perform hot press for baking ceramics, the same problem of pollution as mentioned above may come out.